The present invention pertains to a novel method and apparatus used to establish a perimeter lined, generally horizontally directed drill hole in a variety of soil formations.
In a known method of this type, see, e.g., journal "Erdol, Erdgas, Kohle", Vol 103, No. 12, pp. 531-534, the drill hole is advanced by the use of a directed drilling tool connected to a drill string by an extender section. A mantle tube is tracked along the drill string and the drill hole originally bored by the rotary drill bit of the directed drilling tool is re-bored. As soon as the front end of the mantle tube connects with the directed drilling tool, a section of the drill hole is drilled ahead with the directed drilling tool and then the mantle tube is tracked again. In this manner, both the drill string, including the directed drilling tool, and the mantle linkage are alternately advanced along an imaginary, predetermined line defined as the drill hole axis.
This method of alternatively tracking the mantle tube requires considerable drilling effort due to the re-drilling and produces drill holes with a diameter considerably larger than the rated diameter of the rotary drill bit. Furthermore, the accuracy of this drill head leaves much to be desired since a directed drill tool with an unstabilized bent housing including a deep hole motor is braced directly against the drill hole wall. Particularly in soft formations, e.g. sandy soils, the direction of the advance is quite imprecise and frequent direction corrections are needed to keep the advance of the drill hole near the desired, specified line. Additionally, the removal of fines becomes increasingly difficult as the distance of the drill bit from the front end of the mantle tube increases. These problems are magnified in soft formations and in formations with heavy ground water volume.